1. Field of the Invention
The present invention relates to recovery of mineral values from hot geothermal brines and, more particularly, to an improved brine recovery system which avoids precipitation and scaling in the well and surface recovery equipment.
2. Description of the Prior Art
The utilization of geothermal energy has been, for the most part, concerned with recovery of and utilization of steam to generate electricity by means of a steam turbine driven generator or, where there is a demand for heat in the immediate vicinity of the field, recovered steam or hot water can be employed for space, process, road surface or agricultural heating as appropriate. Where the development of geothermal steam is technically and economically feasible, in an area of significant geothermal discovery, often a public or private electric utility must be induced to buy and distribute the power. To overcome the natural tendency of electric utilities to use conventional fossil or nuclear fuel sources for scheduling power production some years in advance, geothermal suppliers must be willing and able to harness steam in sufficiently large quantities to make reasonable reliance on supply and/or relative costs attractive to the utilities.
Another aspect of geothermal resource development which may hold greater initial promise, in areas similar to the Salton Sea in California is the value of minerals which are present in the geothermal brines in enormous quantities. Current activities are devoted to economic disposal of bitterns associated with the operation and separation of other minerals from the mineral-rich brine. In the Imperial Valley area, the brine is known to contain potassium, lithium, sodium, calcium, strontium, molybdenum, cesium, boron, copper, gold, lead, zinc, silver, iron and manganese in quantities of a theoretical value per well-day of $5,000 for potash, $280 for silver, $480 for borax, $50 for copper, $125 for lead, $650 for zinc, and $30,000 for lithium. Yet technical problems persist relating to corrosion, disposal of bitterns, the separation of the minerals from the brine and other physical problems.
Geothermal operations are somewhat of a gamble at this point--not only as to the likelihood of finding a sufficient quantity to be marketable for power only but also as to finding a market. Well drilling costs range from $35,000 to $400,000. The development of a reliable process for brine extraction under conditions minimizing the rapid build-up of solids resulting in the plugging of lines, nozzles, etc., could make marketing minerals a more deliberate and definable operation than sole reliance on marketing power. This would attract companies interested in developing minerals primarily and energy secondarily from such deposits.
Experience with a flash process as at Niland, Calif. in which geothermal brines were processed at successively lower temperatures and pressures indicates that long periods of operation can be logged before shutdown of the facility for inspection and maintenance. Samples of the brine, suspended solids, and scale deposits were analyzed for bulk composition and mineralogy, but correlation with temperature and pressure was vague.
In general, a galena-rich scale was observed in valves and piping ahead of the first-stage separator, becoming much less abundant beyond that point. The bulk of the scale consisted of an increasing proportion of an iron-rich amorphous silica with process progression. Trace crystalline phases, such as akaganeite, kutnahorite, annd shpalerite and/or wurtzite have been detected or are strongly suspected in the scales. Halite and sylvite are found primarily as a result of numerous facility shutdowns and subsequent cooling of the retained brine in the equipment. Barite is frequently present when irrigation water (used for priming the wells) containing SO.sub.4 is mixed with the brine. Calcite and/or argonite are also found whenever separated steam condensate in contact with CO.sub.2 is recombined with the brine. Corrosion products, magnetic and hematite, form an integral part of the scale adjacent to steel walls. The scales were layered with bands of widely differing material. The color, texture, hardness and thickness of the deposits also varied considerably.
Research sponsored by U.S. Energy Research and Development Administration on scaling characteristics of these brines demonstrated that temperature was believed to be the dominant phenomenon (Modeling The Temperature-Dependent Scale Accumulation From Geothermal Brine, R. C. Schroeder; UCRL-52145). Another report under the same contract indicated the influence of mineralogical chemical composition of brine in scaling and that the amount and rate of deposition of scale is dependent on the salinity of the brine (Scaling Characteristics In The Geothermal Loop Experimental Facility At Niland, California, Roland Quong; UCRL-52162).